1. W. L. Bragg, C. Gottfried and J. West, Z. Krist., 77, 255 (1931)
2. C. A. Beevers and M. A. S. Ross, Z. Krist., 97, 59 (1937)
3. J. T. Kummer, Prosr. Solid State Chem., 2 141 (1972)
4. J. T. Kummer and Neill Weber, US Patent No. 3,404,035 and 3,404,036, October 1, 1968
5. C. R. Peters, M. Bettman, J. W. Moore and M. P. Glick, Acta Cryst., B27, 1826 (1971)